Genetic irony beyond haemochromatosis: clinical effects of HLA-H mutations

THE LANCET

COMMENTARY

COMMENTARY

Genetic irony beyond haemochromatosis: clinical effects of HLA-H mutations See p 321 Until about 20 years ago the disease then known as bronze diabetes or idiopathic haemochromatosis was considered to be an interesting but rare disorder of iron metabolism. Only in the middle 1970s,1 when transmission of the disease was found to be linked to HLA-A and HLA-B did its hereditary nature come into sharp focus. Now usually designated hereditary haemochromatosis, this disorder is arguably the most common disease-producing genetic disorder of Europeans; up to 5 per 1000 are homozygous for this disease. Given the importance of hereditary haemochromatosis and knowing that the gene was closely associated with the HLA genes on the short arm of chromosome 6, several groups, including our own, have attempted to find the disease-producing gene. In 1996 a gene designated

HLA-H, believed to be the gene mutated in hereditary haemochromatosis, was cloned by Feder et al.2 Despite some initial scepticism, their observations were almost immediately confirmed by two groups in the USA3,4 and shortly thererafter by investigators in Australia5 and France.6 In today’s Lancet Roberts et al analyse the HLA-H gene to show clearly for the first time that there is a strong association between hereditary haemochromatosis and another disease long known to be related to iron loading, sporadic porphyria cutanea tarda. Two mutations of HLA-H have been identified, cDNA nucleotide 845G➔A (845A; Cys282Tyr) and 187 C➔G (His63Asp). The compelling data showing the association of these mutations with hereditary haemochromatosis and porphyria cutanea tarda are summarised in the table. The relation between the Cys282Tyr mutation and haemochromatosis is obvious, but that of the His63Asp mutation to iron HLA-H alleles in health and disease loading is much more subtle. The nt 845 nt187 Number of cases ( %) His63Asp mutation is no more frequent (G A) (C G) At risk Haemochromatosis PCT Normal in patients with porphyria cutanea tarda controls or haemochromatosis than in the normal population. However, comparison of 7 ( 17·0) 1 ( 0·2) A/A C/C 0 354 ( 85·1) the raw population frequency in A/A C/G 0 0 0 0 haemochromatosis patients with that in A/A G/G 0 0 0 0 the normal population is not appropriate 8 ( 19·5) 38 ( 8·8) A/G C/C 1 3 ( 0·7) because the Cys282Tyr mutation is in 3 ( 7·3) 9 ( 2·0) A/G C/G 1 19 ( 4·5) complete linkage disequilibrium with A/G G/G 1 0 0 0 the His63Asp mutation—ie, no chromosome with the Cys282Tyr 13 ( 31·7) 270 ( 62·4) G/G C/C 2 22 ( 5·3) mutation carries the H63D mutation. 9 ( 22·0) 100 ( 23·1) G/G C/G 2 15 ( 3·6) Thus, the only chromosomes “at risk” 2 ( 2·4) 15 ( 3·5) G/G G/G 2 3 ( 0·7) for the His63Asp mutation are those without the Cys282Tyr mutation. As 41 ( 100) 433 ( 100) Total 416 ( 100) shown in the table, only 102 *Number of chromosomes at risk of 187G mutation. chromosomes in 416 patients with Data for patients with haemochromatosis from refs 2,3,5,6; normal controls from Rober ts et al haemochromatosis were “at risk” for the and from refs 2,3 His63Asp mutation. Among these, the frequency of the His63Asp mutated 187 845 allele is 39·2%, considerably higher 187 845 187 845 C G than the 17·0% in the general C A C A population. More important, of the C A 22 chromosomes at risk in C A G G haemochromatosis patients G G heterozygous for Cys282Tyr, an HLA-H haplotypes in Most (?) develop iron~ 1·5% develop ironimpressive 19 (86%) contained the human population storage disease storage disease His63Asp mutation. Tight linkage with

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THE LANCET

COMMENTARY another mutation cannot be ruled out, of course, but Feder et al found no other candidate genes in a 250 kb region surrounding the gene, and a cause-and-effect relation is by far the most likely. Thus the His63Asp mutation seems to cause haemochromatosis when inherited together with the Cys282Tyr mutation. The penetrance of the compound heterozygous state seems to be very low; on the basis of Hardy-Weinberg calculations it appears that only about 1% develop clinical haemochromatosis.2,3 As emphasised by the studies of Roberts et al, much remains to be learned about the range of clinical effects of the HLA-H mutations. Until the discovery of this gene it was very difficult to assess the effect of the heterozygous state. Although 10 to 15% of the population are heterozygotes, they could be identified accurately only through linkage studies when a proband for the disease was a family member.7 Since the homozygous state is detrimental, it is likely that heterozygotes enjoy a selective advantage. This advantage would not need to be very large for a high gene frequency to be achieved; although the disease is a serious one, its late clinical onset, usually after the age of 45, minimises its impact on the gene frequency. An obvious advantage would be protection against iron deficiency. As the studies of Roberts et al now show, morbidity may also result. It has previously been suggested that the heterozygous state may adversely affect the course of other diseases, such as hereditary spherocytosis, haemoglobinopathies, or chronic hepatitis, and that heterozygotes for hereditary haemochromatosis may be at increased risk of diabetes, certain malignancies, and coronary artery disease. As pointed out by Roberts et al, previous data regarding the role of haemochromatosis in porphyria cutanea tarda have been contradictory. Now, for the first time, the tools to test some of these associations have become available. The study by Roberts et al is the first to show convincingly a morbid effect of the heterozygous state. Their studies also demonstrate the existence of homozygotes for the Cys282Tyr mutation who do not present with classic haemochromatosis. Early diagnosis of haemochromatosis is vital because the disease is so easily treated by phlebotomy to remove iron, thus preventing the cirrhosis, diabetes, arthritis, and cardiomyopathies that characterise the full-blown disorder. Our armamentarium for the diagnosis of haemochromatosis and for understanding of the heterozygous state has been greatly strengthened by the discovery of the HLA-H gene.

Ernest Beutler Department of Molecular and Experimental Medicine, Scripps Research Institute, La Jolla CA 92037, USA 1

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5 6 7

Simon M, Pawlotsky Y, Bourel M, Fauchet R, Genetet B. Himochromatose idiopathique: Maladie associie `l'antighne tissulaire. Nouv Presse Med 1975; 4: 1432 Feder JN, Gnirke A, Thomas W, et al. A novel MHC class I-like gene is mutated in patients with hereditary haemachromatosis. Nat Genet 1996; 13: 399-408. Beutler E, Gelbart T, West C, et al. Mutation analysis in hereditary hemochromatosis. Blood Cells Mol Dis 1996; 22: 187-94. Calandro L, Thorsen T, Barcellos L, Griggs J, Baer D, Sensabaugh GF. Mutation analysis in hereditary hemochromatosis. Blood Cells Mol Dis 1996; 22: 194A-94B. Jazwinska EC, Cullen LM, Busfield F, et al. Haemochromatosis and HLA-H. Nat Genet 1996; 14: 249-51. Jouanolle AM, Gandon G, Jezequel P, et al. Haemochromatosis and HLA-H. Nat Genet 1996; 14: 251-52. Bulaj ZJ, Griffen LM, Jorde LB, Edwards CQ, Kushner JP. Clinical and biochemical abnormalities in people heterozygous for hemochromatosis. N Engl J Med 1996; 335: 1799-805.

Vol 349 • February 1, 1997

Pre-eclampsia: a case of nerves? Early scholars, having made some very accurate descriptions of pre-eclampsia/eclampsia, recognised an association with proteinuria, raised the notion that this disorder is due to a toxin, and realised that delivery was the only cure,1 would be disappointed to watch us approach the end of this century with few further advances. We know now that magnesium sulphate is a better agent than phenytoin for preventing eclampsia; that several agents are safe and effective antihypertensives in pregnancy; and that aspirin seems to prevent preeclampsia in only a few circumstances.2 We do understand the overall pathophysiology to a point that permits better maternal monitoring,3 which alerts us to when delivery is necessary, before a catastrophic maternal event. Neonatologists have contributed perhaps the most to the management of pre-eclampsia by giving premature or growth-retarded babies an excellent chance of survival and thus making early delivery an option. Why such slow progress in an era when knowledge of many disorders has been unravelled with remarkable speed? First, pre-eclampsia seems to be a uniquely human disease and animal models are, so far, inadequate. Second, pre-eclampsia is caused somehow by the placenta—only removal of the placenta cures this disorder—and studying this organ during human pregnancy is almost impossible. Third, pre-eclampsia manifests variably in different women—eg, liver or platelet abnormalities may dominate in one woman whilst neurological abnormalities or fetal growth retardation may dominate in another. These factors make research into the pathophysiology of preeclampsia dauntingly difficult. Against this background, focused studies of the pathophysiology of pre-eclampsia, such as that of Schobel and colleagues,4 are welcome. These researchers measured sympathetic nervous system activity by microneurography in the peroneal nerve in nine pre-eclamptic women and found greater resting (but not stimulated) electrical activity than in well-selected control groups, including normotensive pregnant and non-pregnant women and seven hypertensive non-pregnant women. This heightened sympathetic nervous system activity, measured as bursts per 100 heartbeats, returned to normal postpartum in parallel with falling blood pressure. The study was executed carefully and the results are clear but, as is often the case in pre-eclampsia research, the interpretation is difficult. Resting and stimulated heart rate were similar in pre-eclamptics and normal pregnant women so it seems that sympathetic activity was altered in peripheral nerves but not in the heart, where perhaps it was balanced by increased vagal tone. The effects of vasodilators and plasma-volume contraction in these subjects also cannot be discounted—ie, the enhanced sympathetic nervous system activity may be a secondary phenomenon. The authors make a cautious claim that blood pressure changes in pre-eclampsia “are mediated, at least in part, by a substantial increase in sympathetic vasoconstrictor activity.” This interpretation of the findings remains to be tested directly but is supported by the knowledge that agents that lower central or peripheral sympathetic nervous system activity (such as methyldopa, ␤-adrenergic blockers, and epidural anaesthesia) lower the blood pressure in pre-eclampsia, as they do in other forms of 297